Phase characterization of iron/manganese fischer-tropsch catalysts: effect of compositon and reduction conditions

The bulk phases of Fe-Mn catalysts after calcination, reduction and synthesis were investigated by Mo˝ssbauer spectroscopy. After calcination at 500°, the iron rich material exists as a single phase solid solution —(Fel-xMnx)203 whereas the Mn-rich material is a mixture of —(Fe1-xMnx)203 and —(Mn1-yFey)203. Consequently,the Mn-rich catalyst ends up as an inhomogeneous conglomerate oT five phases after synthesis. The oxides at all stages of reduction and synthesis have Mn and Fe interdissolved, but the metal carbides have no detectable Mn-alloying. The phase composition after 200 h of Fischer-Tropsch synthesis depends on the reduction temperature. Reduction temperature of 400° leads to complete reduction and to large amounts of carbide and small amounts of spinel after synthesis. Further, the carbide is mainly χ-Fe5C2 and small amounts of O(œ')-Fe2C. These trends reverse when the reduction is carried out at 300°.